With rapid development of broadband services, stricter requirements are imposed on the transmission bandwidth. DWDM is a technology for increasing bandwidth on the existing optical network. This technology transmits multiple channels of signals of different wavelengths in one fiber based on DWDM, thus enhancing the transmission capability of a single fiber. When the number of fibers is limited, a Wavelength Division Multiplexing (WDM) system is one of the methods for increasing the transmission capacity efficiently.
In a DWDM system, with the increase of the transmission channels, the gap between channels is decreasing. Therefore, the control of the central frequency deviation is essential. In order to let the spectral energy of all channel signals fall within the passband of the corresponding optical demultiplexer, the central frequency deviation needs to be controlled within a certain range within the life span of the optical source while the optical source undergoes the ambient temperature and humidity (or temperature dependence) fluctuations. For the optical modules which are typically in optical networks having utilizing a 100 GHz or above channel spacing, the central frequency deviation may be controlled through constant temperature and power control. For the optical modules used in optical networks utilizing a 50 GHz or below channel spacing optical network, stricter requirements are imposed on the wavelength stability and precision control. Accordingly, the central frequency deviation may be controlled through wavelength locking. The wavelength locking mode in the prior art is outlined below.
FIG. 1 shows a structure of the first implementation mode in the prior art. An optical module includes: a feedback controller 110, a laser transmitter 120, and a wavelength locking apparatus 130.
In the optical signal emitted by the laser transmitter 120, a small portion of light enters the wavelength locking apparatus 130. The wavelength locking apparatus 130 includes a wavelength locker 131 and a processing control module 132. The wavelength locker 131 receives input optical signals, and outputs two channels of signals to the processing control module 132, where one channel of signals are a branch of the input signals and is used as reference signals that are unprocessed, and the other channel of signals are the signals that are output after standard grid comparison. The processing control module 132 controls wavelength drift according to the deviation of the received two channels of signals, and outputs the feedback signals to the feedback controller 110. The feedback controller 110 controls the laser transmitter 120 so that the output wavelength of the laser transmitter 120 remains stable. The feedback controller 110 is capable of controlling wavelength deviation by controlling the laser mandrel temperature and power.
The mode in FIG. 1 provides wavelength stability, but sets the wavelength locking apparatus 130 in the optical module. In this way, each laser needs to use a wavelength locking apparatus 130 separately, thus increasing the cost of each optical module. Because a DWDM system generally contains many optical modules, the overall cost increase using this approach may be considerable.
FIG. 2 shows a structure of the second implementation mode in the prior art. In this implementation mode, each optical module contains only a feedback controller and a laser transmitter, for example, optical module 160a to optical module 160n in FIG. 2. The optical signals output from the optical modules are combined by a multiplexer 150 into one optical signal for outputting. The optical signal is demultiplexed by a demultiplexer 140, and a small portion of the demultiplexed optical signal is fed into the wavelength locking apparatus 130. The wavelength locking apparatus 130 includes a wavelength locker 131, a processing control module 132, and a wavelength selecting module 133. The wavelength selecting module 133 receives the optical signal input by the demultiplexer 140, selects the wavelength to be locked among the optical signals of different wavelengths according to the preset policies, and sends the optical signal corresponding to the wavelength to the wavelength locker 131.
Afterward, the wavelength locker 131 outputs two channels of signals to the processing control module 132. The processing control module 132 outputs a feedback signal to the feedback controller in the optical module corresponding to the locked wavelength. Therefore, the laser transmitter in an optical module is controlled and the wavelength output by the optical module remains stable. This processing is exactly the same as the processing in the corresponding part in FIG. 1. In this way, the wavelength selecting module 133 selects the optical signal of each wavelength consecutively so that all wavelengths are selected and locked.
The mode illustrated in FIG. 2 makes multiple optical modules share a wavelength locking apparatus, but the apparatus needs to include a separate wavelength selecting module which is costly. With respect to cost-efficiency, this solution is not optimal. Moreover, the wavelength selecting module is composed of mechanical and electrical units and optical components, and may take a long time to select a wavelength from a beam of optical signals. The mechanical and electrical units can be used for a limited number of times, and have relatively low reliability. In order to meet the life span requirements of telecom equipment, a long locking gap has to be applied, resulting in lower locking precision.
FIG. 3 shows a structure of the third implementation mode in the prior art. The structure of the third implementation mode is almost the same as the structure shown in FIG. 2 except for an additional low-frequency scrambling module 170 which is configured to add a low-frequency modulation signal of different frequencies (f1-fn) to the laser transmitter in each optical module. Generally, the modulation frequency is less than dozens of kilohertz, the Inter-Frequency Gap (IFG) of different wavelengths is typically in the range of 100 KHz-1 KHz, and the modulation amplitude is 1%-5%. Accordingly, the wavelength selecting module is removed from the wavelength locking apparatus 130 and a wavelength extracting module is added instead. The processing of the wavelength locking apparatus 130 may include: the wavelength locker 131 receives the optical signal input by the demultiplexer 140, and outputs two channels of signals to a wavelength extracting module 134. The wavelength extracting module 134 performs digital signal processing, resolves different wavelengths according to different modulation signal frequencies, and transmits the wavelength to be processed to a processing control module 132. The processing control module 132 judges the deviation between the received wavelength and the standard wavelength, and outputs a feedback signal to a feedback controller in the optical module corresponding to the locked wavelength. Therefore, the laser transmitter in the optical module is controlled, and the wavelength output by the optical module remains stable. This processing is exactly the same as the processing in the corresponding part in FIG. 1. In this way, the wavelength extracting module 134 demodulates and resolves all the wavelengths consecutively so that the wavelengths are selected and locked.
For the implementation mode shown in FIG. 3, different low-frequency modulation may need to be performed for different wavelengths simultaneously. Consequently, the low-frequency scrambling apparatus is complicated, and needs to be implemented through the technologies such as decimal frequency division. Moreover, the gap between different perturbation frequencies is small, and interference exists between the frequencies. Therefore, higher requirements are imposed on filtering and extracting of the scrambled signals, the whole apparatus is therefore more complicated, and the reliability of locking wavelengths is reduced.